The pathophysiology of acute renal failure, the sudden cessation of kidney function in a patient without preexisting kidney disease, is poorly understood. Many of the physiologic abnormalities which have been postulated to play a role in this common, costly, and often lethal disease would be expected to have a morphologic counterpart. We will try to identify renal morphologic changes which are present in animals with severe post-ischemic acute renal failure and absent in animals which were subjected to the same ischemic insult but did not develop severe renal failure because they were given a drug which had a protective effect. Such studies, carried out using standard light microscopy, and scanning electron microscopy, will provide clues to the mechanism of pharmacologic protection against acute renal failure and to the pathophysiology of acute renal failure itself. Intravital microscopy will be used to find out which morphologic differences between treated and untreated groups precede differences in creatinine clearance. The relationship between renal pathology and renal insufficiency will also be studied in experimental gentamicin nephrotoxicity, in the post-obstructive kidney, and in acute (bacterial) interstitial nephritis. Human renal biopsy material will be used as a point of reference in the interpretation of these animal studies.